Thermophotovoltaic power generation is a technique of converting thermal radiation to electric power using a photoelectric conversion cell. The thermophotovoltaic power generation is attracting attention as a power generating technology that promises highly efficient power generation by controlling a radiation spectrum, allows to use a variety of thermal sources, and provides high energy density per unit weight.
FIG. 9 illustrates a basic configuration of thermophotovoltaic power generation. The configuration includes: converting heat generated from combustion equipment or heat generated by condensing solar light to infrared radiation by an emitter 4; allowing the infrared light to enter a photoelectric conversion cell 5; and converting the infrared light to electric power. While a variety of emitter materials are reported, the necessary condition for increasing the efficiency of thermal photovoltaic power is narrowing down the infrared spectrum to a wavelength matched with the photoelectric conversion cell.
FIG. 10 illustrates a configuration of a thermophotovoltaic power generation device where a photonic crystal 6 is provided on the surface of the emitter. For example, as described in PTL 1, there has been suggested a control of an infrared spectrum using a photonic crystal where many cavities are formed on metal. However, a metal emitter has a drawback that it is degraded by oxidization or recrystallization upon being used at a high temperature.
Further, as illustrated in FIG. 11, there has been suggested a configuration with an optical filter between an emitter and a photoelectric conversion cell. However, when the optical filter 7 is used, it is not possible to increase efficiency unless the emitter reflects/absorbs infrared light other than the wavelength adaptable to the photoelectric conversion cell. Thus, the configuration has drawbacks that the device structure becomes complicated and realizing a high heat-resistant filter is difficult.
Therefore, emitters using ceramic that has excellent oxidation resistance and heat resistance have been actively studied and developed. PTL 2 reports a thermophotovoltaic device that includes: a burner that combusts fuels; a radiant burner screen that has a substrate including a porous or perforated material with heat resistance; and a photovoltaic cell arrangement. The radiant burner screen is coated with a chemical compound including a rare-earth element, thereby forming an infrared emitter. As for the coating chemical compound, ytterbium substituted yttrium aluminum garnet (Yb: YAG) is reported.
Chemical compounds having rare-earth elements are known as providing a selective thermal radiation at a wavelength corresponding to the absorption of 4f electron transition of the rare earth ions. Therefore, there have been reported cases of ceramic emitters using rare earth aluminum garnet. Such reports include, for example, NPL 1 and NPL 2.
NPL 1 reports an emitter that is configured by forming, on SiC ceramic, a composite coating of alumina or zirconia fibers and erbium aluminum garnet, Er3Al5O12 (ErAG), that has 50% or more porosity and 50 to 500 μm in thickness. FIG. 12 illustrates a thermal radiation spectrum of the ErAG composite and SiC ceramic at 1050° C. Although a selected wavelength radiation can be observed around a wavelength of 1600 nm, there is a drawback that the radiation intensity is relatively smaller compared with SiC that has emissivity of 0.9.
Further, NPL 2 reports an emitter that selectively uses melt growth composite materials made of alumina and rare-earth aluminum garnet where Er and Yb are selected as the rare-earths. FIG. 13 illustrates the wavelength dependency of the emissivity of such emitters. There is a drawback that the emissivity of around 0.7 even at a peak value is relatively smaller compared with SiC whose emissivity is 0.9 thorough the entire wavelength range. Further, when the wavelength selectivity of emissivity is defined by the ratio of the emissivity at a peak wavelength to the emissivity at a wavelength of 1750 nm, the ratio is 1.7 for a composite of YbAG and alumina, 1.5 for a composite of ErAG and alumina, which are not very good results.